Impedance transformer for wave guides



Decca 9, 1947. A. G. Fox

IMPEDANCE TRANSFORMER FOR WAVE GUIDES Original Filed July so, 1942 INVEN TOR A. 6. FOX

A TTORNE 1 Patented Dec. 9, 1947 UNITED STATES OFFICE 2,432,094 IMPEDANCE TRANSFORMER we were GUIDES- Arthur Gardner Fox, Red Banke N. J assignorto Bell Telephone Laboratories, Incorporated; New York, N. Y., a corporation or New York Original application July 30;, 1912, serial no;

452,851. Divided and this application September 7, 1945, Serial No. 614,935

13 Claims. i

This invention relates to guided electromagnetic wave transmission and more particularly to impedance transformers for wave guides.

The principal object of the invention is to interconnect without appreciable impedance mismatch two rectangular wave guides which difier in characteristic impedance and in one crossse'ctional dimension; A further object is to neutralize the reactance at the ends of a quarterwave impedance transforming section used to connect two such guides.

A'uniform metallic sheath with or without a. dielectric filler will serve as a guide for suitable electromagnetic waves. In cross-section the sheath may be circular, rectangular, or of other shape. For all frequencies above a minimum, known as the cut-off frequency, the guide acts like a transmission line and has a specific propagation constant and characteristic impedance. For any particular frequency there are an infinite number of cross-sectional sizes and shapes of guide which will have the same characteristic impedance.

Shunt reactive elements may be obtained by placing partial obstructions across the wave guide,- For example, a shunt reactive element for dominant transverse electric waves may be obtained by using a transverse metal partition having a slit therein which extends substantially from one side to the other. If the slit is perpendicular to the direction of polarization of the electric field the element is primarily capacitive, and if parallel with the field the element is primarily inductive. rectangular opening in the partition may be proportioned to provide parallel resonance, that is, a high shunt impedance,

For a rectangular guide a In accordance with the present invention two rectangular wave guides having cross-sections which are the same in one dimension but diiTer in the other dimension are matched in impedance by interposing a quarter-wave section of rectangular guide having a characteristic impedance which is approximately the geometric mean of the characteristic impedances of the guides to be connected. In the embodiment shown the equal cross-sectional dimensions of the guides are in the direction perpendicular to the electric field of the dominant transverse electric waves. In this case the proper characteristic impedance for the interposed section may be obtained with a cross-section the dimension of which in the direction perpendicular to the electric field is the same as the common dimension of the connected guides and the other dimension of which is approximately equal to the geometric ean of the unequal cross-sectional dimensions of the guides. In the preferred form of the transformer thereactanc'e at each end of the quarter-wave section associated-'with-thechange in cross-section is neutralized by the introduction of a reactance of the same magnitude but of opposite sign. As shown, these neutralizing reactanc'es 'are in the form of-fiaps which constrictthe junction apertures the direction of the equal transverse di mensions and thus conve'rt the apertures into parallel resonant shuntxreactances. I

nusis'a division of application Serial No. 452,851 filed July-30, 1942;. p V

The nature of the invention will be more fully understood from the following detailed description'a'ndby reference to theaccompanying-drawing; the single hea t of which is a perspective view, partly" cutaway, of-awave guide transformer in accordance-withthe invention.

The figureshowswhat may be termed 'aneuitralizedquarter-wave transformer for connecting two wave guides 58 and 6! which difier in size and in characteristic impedance, and are assumed to be carrying dominant transverse elec tric'w'aves'with'theelectric field E polarized inthe direction indicated by nea'rrew, Ihe guides 60 and '61 have rectangular cross s'ections of the same width M- butdiifer in the cross-sectional dimensions Ii andirwhich are parallel to the direction of the e1ectric-fie1d E; The guides 60 and Iii-are connected by an intermediate section of rectangular guide 62, also of width M, which has a'length N-approximately equal'to a quarter wavelength; or an oddrnultipl'ethereof; at the midterm frequency to be transmitted. The characteristic impedance of the section 62' is made approximately the geometric mean of those of the guide fifl and 3| by making its height I2 equal to V1113. Since the cross-section of the system is changed in the direction of the electric field E at each of the junction points 63 and 64, the junctions appear like shunt capacitive reactances. In order to neutralize these capacitive reactances the junction 63 is constricted in the magnetic direction by the addition of the flaps E5 and 66 and the junction 64 is likewise constricted by the flaps 61 and 68. These flaps are made of proper width P to introduce a shunt inductive reactance which, at the mid-band frequency to be transmitted, is equal in magnitude but opposite in sign to the associated capacitive reactance. In this way each junction 63 and 64 is converted into a parallel resonant shunt reactance.

What is claimed is:

1. In combination, two rectangular wave guides differing in characteristic impedance and an interposed rectangular section of wave guide, said guides and said section having the sametransverse dimension in the direction perpendicular to the electric field of the dominant type of wave to be transmitted, said guides differing in their other transverse dimensions, the other transverse dimension of said section being approximately equal to the geometric mean of said other transverse dimensions of said guides, and said section having a length approximately equal to a quarter wavelength at the mid-band frequency to be transmitted, and reactive means at each end of said section for neutralizing the reactance at that point.

2. The combination in accordance with claim 1 in which all of said wave guides have the same type of core material.

3. The combination in accordance with claim 1 in which all of said wave guides are of the airfilled type.

4. The combination in accordance with claim 1 which said reactive means include means for constricting he op g between said section and one of said guides in the direction perpendicular to said electric field.

5. In combination, two rectangular wave guides differing in characteristic impedance and a interposed rectangular section of wave guide, said guides and said section having the same transverse dimension in the direction perpendicular to the electric field of the dominant type of wave to be transmitted, said guides differing in their other transverse dimensions, the other transverse dimension of said section being approximately equal to the geometric mean of said other transverse dimensions of said guides, and said section having a length approximately equal to a quarter wavelength at the mid-band frequency to be transmitted, and means for constricting the openings between said section and each of said guides in the direction perpendicular to said electric field.

6. The combination in accordance with claim 1 in which said reactive means include a pair of oppositely'disposed transverse flaps extending in the direction of said electric field.

7 in combination, two rectangular wave guides differing in characteristic impedance and an interposed rectangular section of wave guide, said guides andsaid section having the same transversedimension in the direction perpendicular to the electric field of the dominant type of wave to be transmitted, said guides differing in their other transverse dimensions, th other transverse dimension of said section being approximately equal to the geometric mean of said other transverse dimensions of said guides, and said section having a length approximately equal to a quarter wavelength at the mid-band frequency to be transmitted, and at each end of said section a pair of oppositely disposed transverse flaps extending in the direction of said electric field.

8. An impedance transformer for interconnecting two rectangular wave guides which differ in characteristic impedance and in one transverse dimension comprising a section of wave guide having a length approximately equal to an odd multiple of a quarter wavelength at the midband frequency to be transmitted and a characteristic impedance approximately equal to the geometric mean of the characteristic impedances of the guides to be connected and reactive means at each 'end of said section for neutralizing the reactance at that point.

9. A transformer in accordance with claim 8 in which said reactive means comprise means for constricting the opening between said section and one of said guides.

10. A transformer in accordance with claim 8 in which said reactive means comprise a pair of oppositely disposed transverse flaps.

11. A transformer in accordance with-claim 8 in which said reactive means comprise means for constricting th openings between said section and each of said guides.

12. A transformer in accordance with claim 8 in which said guides are substantially alike in their other transverse dimension and said reactive means comprise means for constricting the opening between saidsection and one of said guides in the direction of said other transverse {111.-611510'11.

13. A transformer in accordance with claim 8 in which said guides are substantially alike in their other transverse dimensions and said reactive means comprise a pair of oppositely disposed transverse flaps for constricting the opening between said section and one of said guides in the direction of said other transverse dimension.

ARTHUR GARDNER FOX.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,106,769 Southworth Feb. 1. 1938 2,270,416 Cork Jan. 20, 1942 2,253,503 Bowen Aug. 26, 1941 

